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An ultrahot gas-giant exoplanet with a stratosphere


Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere1,2,3. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum4,5. Conversely, if there is a stratosphere—where temperature increases with altitude—these molecular features will be observed in emission6,7,8. It has been suggested that stratospheres could form in highly irradiated exoplanets9,10, but the extent to which this occurs is unresolved both theoretically11,12 and observationally3,13,14,15. A previous claim for the presence of a stratosphere14 remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements3. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.

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Figure 1: Wavelength-integrated white light curve for WASP-121b.
Figure 2: Emission spectrum for WASP-121b.
Figure 3: Temperature–pressure profiles for WASP-121b.

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This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 336792 and is supported by the ERC Horizon 2020 research and innovation programme (grant agreement no. 724427). Support for this work was provided by NASA through grants under the HST-GO-14767 “Panchromatic Comparative Exoplanetary Treasury (PanCET)” programme from the STScI. J.G. acknowledges support from a Leverhulme Trust Research Project Grant. H.R.W acknowledges support from the NASA Postdoctoral Program, administered by Universities Space Research Association through a contract with NASA. M.S.M. acknowledges support from the NASA Exoplanets Research Program. J.K.B. acknowledges support from a Royal Astronomical Society Fellowship. D.E. and V.B. acknowledge the financial support of the National Centre for Competence in Research “PlanetS” supported by the Swiss National Science Foundation (SNSF). A.L.E. acknowledges support from CNES and the French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 “Exo-Atmos”. J.S.-F. acknowledges support from the Spanish MINECO through grant AYA2014-54348-C3-2-R. G.W.H. acknowledges support from NASA, NSF, Tennessee State University, and the State of Tennessee through its Centers of Excellence programme. L.B.-J. and P.L. acknowledge support from CNES (France) under project PACES. P.T. and D.S.A. acknowledge funding from the European Research Council under the European Union Seventh Framework Program: grant 247060-PEPS.

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Authors and Affiliations



T.M.E. and D.K.S. designed the HST observations of WASP-121. D.K.S. and M.L.-M. led the HST Treasury programme, with support provided by all authors. T.M.E. led the HST data analysis with contributions from N.N., H.R.W. and D.D. D.D. proposed and designed the Spitzer observations and analysed the data. D.K.S. led the retrieval analysis. T.K., J.G., M.S.M., A.L.E. and P.T. provided additional theoretical interpretation of the data. R.L. provided molecular absorption cross-sections for the theoretical interpretation. T.M.E. wrote the manuscript along with D.K.S., T.K., M.S.M. and A.L.E. All authors discussed the results and commented on the paper. The author list ordering is alphabetical after M.S.M.

Corresponding author

Correspondence to Thomas M. Evans.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks K. Heng and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Spectroscopic light curves for WASP-121b.

a–d, Raw normalized light curves for each of the spectroscopic channels with photon-noise 1σ error bars (orange and green circles), and best-fit eclipse signals multiplied by linear time trends (grey lines). Vertical offsets have been applied for visual clarity. Labels indicate central wavelengths for each channel. eh, Model residuals after removing the best-fit eclipse signal and linear time trend for each of the spectroscopic light curves in ad, respectively, with photon-noise 1σ error bars (orange and green circles) and stochastic components of the best-fit Gaussian process models (black lines). Note that for most channels, the correlations remaining in the residuals after accounting for a linear time trend are minimal.

Extended Data Figure 2 Model thermal spectrum for WASP-121b broken down by emission source and absorption cross-sections of important molecules.

a, Similar to Fig. 2, showing the HST measurements of the WASP-121b thermal spectrum (brown circles; error bars, 1σ uncertainties). Red line shows the best-fit model (H2O and VO) obtained from the retrieval analysis. Other coloured lines show model thermal spectra generated using the same TP profile as for the best-fit model but with the opacity due to each molecule turned off one-by-one: VO off (blue line) and H2O off (green line). b, Absorption cross-sections for plausible gas-phase absorbers (colour coded, key at top right) across the WFC3 bandpass. Text labels in the key give the rescaling factors that have been applied to each cross-section to fit them on a single vertical axis, with the exception of the H2O cross-section which has not been rescaled. Note that the VO cross-section has been rescaled by a factor of 0.03, which is consistent with the abundance of VO relative to H2O inferred for the best-fit model (Extended Data Table 4).

Extended Data Figure 3 A comparison of models with and without stratospheres.

a, A sequence of TP profiles with and without stratospheres computed using the analytic parameterization of ref. 23. The red line corresponds to a model with a strong stratosphere obtained from the MCMC retrieval analysis and the green line shows a model with a weaker stratosphere. The light blue and dark blue lines show, respectively, models with a decreasing temperature profile and a strongly-decreasing temperature profile. The yellow line indicates the best-fit isothermal model with a temperature of 2,700 K. b, Corresponding contribution functions averaged over the WFC3 G141 bandpass, indicating the approximate pressures probed at these wavelengths. c, Corresponding thermal spectra, with H2O and VO bands seen in emission for the models with stratospheres (red and green lines) and in absorption for the models with decreasing temperature profiles (light blue and dark blue lines). The isothermal model has a featureless blackbody spectrum (yellow line).

Extended Data Figure 4 Self-consistent models in radiative-convective equilibrium.

a, Similar to Fig. 2, but showing self-consistent models in radiative equilibrium: median H2O and VO abundances obtained from the retrieval analysis (light blue line); H2O abundance set to median value and VO abundance set to the lower value of the 68% credible range (green line); solar metallicity, but without VO and TiO, and with an arbitrary absorber with absorption cross-section κ = 0.02 cm2 g−1 (pink line); and 15× solar metallicity, but without VO and TiO, and with an arbitrary absorber with κ = 0.06 cm2 g−1 (dark blue line). For the last two models, the arbitrary absorber has grey opacity across the 0.43–1 μm wavelength range and is assumed to be distributed uniformly throughout the atmosphere. The best-fit isothermal model with temperature 2,700 K is also shown (yellow line). Brown circles with error bars, HST data with 1σ uncertainties: inset, HST data on a magnified scale. b, Corresponding TP profiles, all of which have a stratosphere across the 10−2 bar to 10−5 bar pressure range. Thick lines indicate the main pressure levels probed across the WFC3 G141 bandpass.

Extended Data Figure 5 Posterior distributions from MCMC retrieval analysis for WASP-121b.

Panels along the diagonal show the marginalized posterior distributions for each parameter of the thermal emission model (κIR, γ1, β, H2O abundance, VO abundance), shown left to right in the columns, and top to bottom in the rows. Solid orange lines show the posterior medians and dashed orange lines show the ranges either side encompassing ±34% of the samples. Panels below the diagonal show the posterior distribution as a function of each parameter pair with increasing probability from light to dark shades. As γ1 controls the optical opacity, there is a strong anti-correlation with the radiation efficiency factor β, where lower radiation efficiency values are compensated by higher optical opacities and vice versa. There is also a strong degeneracy between the abundances of each molecule (H2O, VO) and the parameter κIR which controls the overall atmospheric opacity. Higher abundances and higher values of κIR result in stratospheres at lower pressures and vice versa.

Extended Data Figure 6 ATMO retrieval code applied to WASP-43b thermal emission data.

a, Black data points show the measured thermal emission data for WASP-43b taken from ref. 48, composed of HST/WFC3 and Spitzer/IRAC observations. Vertical error bars give 1σ uncertainties and horizontal error bars give photometric bandpasses. The red line shows the best-fit spectrum obtained from an ATMO retrieval analysis, with shading indicating the regions encompassing 68%, 95% and 99.7% of the MCMC samples. The inset shows a magnified view of the HST data. The retrieved volume mixing ratio (VMR) for H2O is in good agreement with the published value obtained using the CHIMERA retrieval code2,3 and the ATMO retrieval achieves a better fit to the data as measured by the minimum χ2. b, Best-fit TP profiles retrieved with ATMO (solid blue line) and CHIMERA (solid green line). The two analyses agree to within the 68% credible ranges (blue and green dashed lines).

Extended Data Table 1 Parameter values for the WASP-121b white light curve fit
Extended Data Table 2 WFC3 spectroscopic eclipse depths for WASP-121b
Extended Data Table 3 Comparison of different retrieval analyses for WASP-121b
Extended Data Table 4 MCMC retrieval analysis results for WASP-121b

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This file contains an archive of ASCII files containing reduced HST data products and models used in the paper. (ZIP 329 kb)

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Evans, T., Sing, D., Kataria, T. et al. An ultrahot gas-giant exoplanet with a stratosphere. Nature 548, 58–61 (2017).

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